1. Field of the Invention
The present invention relates to a method of forming a CVD titanium film, which is suitable for use in a semiconductor device, and particularly to a method of depositing a titanium film on a cobalt silicide film.
2. Description of the Related Art
With micro-fabrication of semiconductor elements, contact holes are reduced in diameter and contact hole portions increase in aspect ratio. With a view toward forming a desired contact or adhesive layer (e.g., titanium nitride TiN) at a sidewall portion of the scaled-down contact hole or at its bottom by a sputtering method, the deposited thickness of adhesive layer must be increased. Since an increase in the thickness of a sputtering film having an overhung shape narrows the diameter of a contact entrance as shown in FIG. 1, the subsequent embedding of a tungsten film (W) in each contact hole by a chemical vapor deposition (CVD) method becomes difficult. Therefore, a CVD method capable of obtaining excellent coverage has been expected as an alternative to the sputtering method.
In order to form a titanium nitride film by the CVD method, organic or inorganic titanium tetrachloride (TiCl4) such as tetrakisdimethylamino titanium (TDMAT), tetrakisdiethylamino titanium (TDEAT) or the like is used as a raw material gas. However, the organic raw material is high in cost and poor in coverage too under present circumstances. Since carbon (C) is contained in the film as an impurity, there is the demerit of increasing specific resistivity up to ten times that of the sputtering film. Therefore, titanium tetrachloride is generally used in plenty as the raw material gas. Since the deposition of titanium by the organic raw material gas is difficult, titanium tetrachloride is used even for the formation of a titanium (Ti) film.
FIG. 2 shows the dependence of the deposited thickness of titanium on silicon (Si), a silicon oxide film (SiO2) and a cobalt silicide (CoSi2) layer on deposition time. The present drawing indicates that the deposition of titanium on the cobalt silicide (CoSi2) layer is slow as compared with deposition on the other underlying beds. This phenomenon results from the mechanism of deposition or growth of the CVD titanium film. When the underlying bed is silicon, the deposited titanium reacts with silicon to form a titanium silicide (TiSix) layer. The resultant titanium silicide layer is etched by a titanium tetrachloride gas.
However, the titanium deposited on each of the silicon oxide film and the cobalt silicide (CoSi2) layer undergoes etching of the titanium tetrachloride gas, and the finally-deposited film-thickness of titanium is determined according to the balance between deposition and etching. The titanium on the silicon oxide film and the cobalt silicide (CoSi2) layer are different in titanium deposition rate because of the difference in etching rate due to the difference in film quality of titanium. Thus, the problem is to control or restrain the etching of titanium deposited on the cobalt silicide (CoSi2) layer.
The present invention aims to cause an element that reacts with titanium to be contained in the surface of cobalt silicide (CoSi2) or in the cobalt silicide (CoSi2) to thereby form a titanium compound during CVD titanium deposition and realize the deposition of a CVD titanium film at a high deposition rate, which has controlled etching using a titanium tetrachloride gas.